The reduction in size of electronic equipment achieved through the advancement of integrated circuit technology has dictated the development of smaller capacitors with sufficient capacitance for use in coupling and bypass applications. Capacitors using ferroelectric ceramic as the dielectric material can achieve a very high capacitance per unit volume due to its high permittivity. Moreover, ceramic capacitors can be manufactured very economically in a wide range of capacitance values and working voltages.
A particular type of capacitor referred to as a monolithic multilayer ceramic capacitor is fabricated by alternating layers of metal and ceramic material in a simple parallel plate arrangement. The capacity of such a device being a function of the number and surface area of the metallic layers (electrodes), their separation, and the permittivity of the ceramic (dielectric). Literally millions of such devices are provided each year by capacitor manufacturers.
Unfortunately, during the fabrication of such capacitors defects may be formed therein. Defects such as cracked ceramic, voids and delaminations of the layers have been found by destructively sectioning the capacitor after completion of the fabrication steps. Such mechanical defects are not always detected in standard electrical tests resulting in inferior product being placed in the field.
In view of the high production of such capacitors it would be most desirable to have a test procedure capable of detecting such defective capacitors early in the fabrication thereof and removing them from further processing, resulting in substantial cost savings. Additionally, service maintenance costs may be significantly decreased by reducing the number of capacitor field failures.
The present technique for detecting defects in multilayer ceramic capacitors is by sample lot evaluation. A small number of capacitors are withdrawn from a production lot, mounted in epoxy, sectioned and microscopically examined. This procedure is most time consuming and defects are judged by a technician based upon a subjective comparison to a predetermined scale. Typically, less than 100 capacitors out of several hundred thousand are tested using such techniques. Such a small sample may not be statistically significant.
Efforts have been made to replace the sectioning technique with non-destructive methods capable of examining a much larger sample of capacitors. Radiography and ultrasonic techniques have been attempted. However, radiography, as well as ultrasonics are both expensive techniques and not amenable to production line volume testing.
The most promising approach to the problem of finding physical defects in multilayer ceramic capacitors has been Acoustic Emission (AE) testing. AE testing is a well known technique and is described in U.S. Pat. No. 4,086,817 to Jon et al. which issued on May 2, 1978 and which is assigned to the instant assignee. AE may be defined as elastic waves which are characterized by low amplitude, short duration and fast rise time signals which are propogated in a structure as the result of an applied stress. The Jon patent is directed to detecting such signals, and operating thereon, to determine the quality of welds.
The use of AE as it relates to testing ceramic capacitors is based upon the premise that defects, such as cracks and delaminations, will propagate with the release of acoustic energy when an external stress is applied to a defective capacitor. The AE signal would then be detected and processed to determine the extent, if any, of physical defects therein.
Various attempts have been made to determine a stressing configuration capable of causing the defects to propagate in order to produce an AE signal representative of defects in the capacitor under test. Initially thermal shocking of a barium titanate ceramic laminated capacitor was thought to have promise to induce sufficient stress therein. However, experiments using a high sensitivity piezoelectric transducer revealed insignificant AE activity upon contact of the capacitor with a 750 F heated probe or immersion in a 500 F fluid.
A more predictable stress was obtained by bending the capacitor chips in a three-point bending fixture. The peak load (6.0 Kg) was chosen sufficiently below the average fracture strength of the capacitors so that the acceptable capacitors were not broken. However, no correlation was found between the severity of internal defects and the resultant AE signal in the sample tested. It appears that the relatively low load at which a good capacitor can be damaged is a serious disadvantage in this loading scheme.
In order to allow the use of larger loads, a capacitor under test was held in a vertical position and a load applied to the edge thereof, parallel to the laminations, by a steel ram. In this configuration of compression, the capacitor can tolerate much higher loads (e.g., 100 Kg) without damaging a good unit.
This parallel loading configuration resulted in capacitors exhibiting a very high AE signal when the capacitor had extensive internal physical damage therein and a very low AE signal when the capacitors were defect free. However, capacitors yielding a low to moderate AE signal gave ambiguous results for a substantial number of the capacitors tested.
Accordingly, it should be clear that the need exists for a non-destructive technique for unambiguously determining the physical acceptability of laminated ceramic capacitors.